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                                                                   Exhibit 10.11

                             MKS INSTRUMENTS, INC.

                                TENTH AMENDMENT

                               TO LOAN AGREEMENT

     This Tenth Amendment (the "Amendment") dated as of January 1, 2000 concerns
the Loan Agreement dated as of November 1, 1993 (the "Loan Agreement"), between
MKS Instruments, Inc. (the "Borrower") and BankBoston, N.A. (f/k/a The First
National Bank of Boston; the "Lender"), as amended. Capitalized terms used
herein but not otherwise defined shall have the meanings assigned to them in the
Loan Agreement.

     WHEREAS, the Borrower has requested that the Lender agree to change certain
provisions of the Loan Agreement; and

     WHEREAS, the Lender is willing, on the terms, subject to the conditions and
to the extent set forth below, to amend the Loan Agreement to effect such
changes;

     NOW, THEREFORE, the Lender and the Borrower agree as follows:

     Section 1. Amendment of the Loan Agreement.

     (a) Section 1.1 of the Loan Agreement is hereby amended by deleting the
definition of "Consolidated Tangible Net Worth" and replacing it with the
following:

          "Consolidated Tangible Net Worth" shall mean, at any time, the
     stockholders' equity of the Borrower and its Subsidiaries determined in
     accordance with generally accepted accounting principles excluding the book
     amount of all minority interests in Affiliates and any foreign exchange
     translation adjustment, with no upward adjustments due to a reevaluation of
     assets (other than any such upward adjustment as may be required under
     generally accepted accounting principles in connection with the acquisition
     by the Borrower or any Subsidiary of another company or entity) minus the
     following items (without duplication of deductions) appearing on the
     balance sheet of the Borrower and its Subsidiaries:

          (a) the book amount of all assets (including, without limitation,
     goodwill, patents, trademarks, copyrights, organizational expense and
     unamortized debt discount) that would be treated as intangibles under
     generally accepted accounting principles;

          (b) treasury stock; and




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          (c) any write-up in the book amount of any asset or Investment
     subsequent to the Closing Date, resulting from a reevaluation or
     reappraisal thereof from the amount entered in accordance with generally
     accepted accounting principles by the Borrower or any Subsidiary on its
     books with respect to its acquisition of the asset or Investment.

     (b) Section 7.1(d) of the Loan Agreement is hereby amended by deleting the
word "thirty" and replacing it with the words "forty-five".

     (c) Section 8.1 of the Loan Agreement is hereby amended by deleting Section
8.1(a) in its entirety and replacing it with the following:

          (a) Sale of Assets. The Borrower will not, except in the ordinary
     course of business, sell, transfer or otherwise dispose of, to any Person
     any assets (including the securities of any Subsidiary) other than assets
     having an aggregate fair market value less than seven percent of Borrower's
     Consolidated Tangible Net Worth.

     (d) Section 8.1(b) of the Loan Agreement is hereby amended and restated as
follows:

          (b) Mergers, Etc. Neither the Borrower nor any Subsidiary will
     consolidate with or merge into any other Person or permit any other Person
     to consolidate with or merge into it, or acquire all or substantially all
     of the capital stock or assets of any Person, or sell, assign, lease or
     otherwise dispose of (whether in one transaction or in a series of
     transactions) all or substantially all of its assets to any Person, except
     that

               (1) a Subsidiary may consolidate with or merge into the Borrower
          or another Subsidiary; and

               (2) the Borrower or any of its Subsidiaries may acquire all or
          substantially all of the capital stock or assets of any Person or
          consolidate or merge with any Person provided (i) such Person is
          engaged in a line of business substantially similar to one or more of
          Borrower's existing lines of business, (ii) the aggregate purchase
          price liability incurred in any calendar year, including all
          contingent liabilities, when aggregated with all such acquisitions and
          any Investments permitted under Section 8.4(3) in any calendar year
          shall not exceed 25% of Consolidated Tangible Net Worth as of the end
          of the most recent fiscal quarter or, if 80% or more of the purchase
          price is paid in capital stock of the Borrower, 50% of Consolidated
          Tangible Net Worth as of the end of the most recent fiscal quarter and
          (iii) based on a pro forma calculation of the ratios set forth in
          Section 8.7 as of the date such acquisition is closed, assuming
          consolidation of the acquired business with the Borrower for the four
          full fiscal quarters ended immediately preceding such closing and pro
          forma

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          debt and debt service payments based on scheduled principal payments,
          including acquisition borrowings, if any, and pro forma interest on
          total debt at then prevailing borrowing rates, Borrower is in
          compliance with the financial covenants set forth in Section 8.7.

     (e) Section 8.4 of the Loan Agreement is hereby amended and restated as
follows:

               8.4 Investments. Except as permitted by Section 8.1, neither the
          Borrower nor any Subsidiary will make or maintain any investments,
          made in cash or by delivery of property or assets, (a) in any Person,
          whether by acquisition of capital stock, Indebtedness, or other
          obligations or securities, or by loan or capital contribution, or
          otherwise, or (b) in any property, whether real or personal, (items
          (a) and (b) being herein called "Investments") except the following:

               (1) Investments in direct obligations of, or guaranteed by, the
          United States government, its agencies or any public instrumentality
          thereof and backed by the full faith and credit of the United States
          government with maturities not to exceed (or an unconditional right to
          compel purchase within) three years from the date of acquisition;

               (2) Repurchase agreements collateralized by securities of the
          U.S. Government and U.S. Government-sponsored securities;

               (3) Investments in or to any Subsidiary or other Affiliate,
          provided Borrower remains in compliance with Section 8.1(b);

               (4) Investments and obligations issued by the United States
          government, any agency thereof, any state of the United States or any
          political subdivision of any such state or any public instrumentality
          thereof with maturities not to exceed (or an unconditional right to
          compel purchase within) three years from the date of acquisition that
          are rated AA- or higher by at least one nationally recognized rating
          agency;

               (5) Investments and obligations issued by any company (other than
          a bank) with maturities not to exceed three years from the date of
          acquisition with a long-term debt rating of A or higher or a
          short-term debt rating of A1 or P1 by at least one nationally
          recognized rating agency;

               (6) Investments in demand and time deposits with, Eurodollar
          deposits with, certificates of deposit issued by, or obligations or
          securities fully backed by letters of credit issued by (x) any bank
          organized under the laws of the United States, any state thereof, the
          District of Columbia or Canada having combined capital and surplus
          aggregating at least $500,000,000, or (y) any other bank organized
          under the laws of a state that is a member of the European Economic
          Community (or any political subdivision thereof), Japan, the Cayman
          Islands, or

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          British West Indies having as of any date of determination combined
          capital and surplus of not less than $500,000,000 or the equivalent
          thereof (determined in accordance with generally accepted accounting
          principles);

               (7) Shares of money market mutual funds registered under the
          Investment Company Act of 1940, as amended;

               (8) Foreign currency swaps and hedging arrangements entered into
          in the ordinary course of business to protect against currency losses,
          and interest rate swaps and caps entered into in the ordinary course
          of business to protect against interest rate exposure on Indebtedness
          bearing interest at a variable rate;

               (9) Investments in mutual funds (other than money market mutual
          funds) that in the aggregate shall not exceed $5,000,000; and

               (10) Other Investments existing on the Closing Date and listed on
          the Disclosure Schedule.

     (f) Section 8.7 of the Loan Agreement is hereby amended by deleting
subsection (a) and replacing it with the following:

               (a) Tangible Net Worth Test. The Consolidated Tangible Net Worth
          as of the end of each fiscal quarter of the Borrower shall not be less
          than the sum of (i) $100,000,000, plus (ii) 50% of Consolidated Net
          Income (excluding losses) minus (iii) all Sub S Dividends paid between
          January 1, 2000 and September 15, 2000 in an aggregate amount not to
          exceed $6,000,000, for each consecutive fiscal quarter of the Borrower
          beginning with the quarter ending December 31, 1999, on a cumulative
          basis.

     Section 2. Representations and Warranties. The Borrower hereby represents
and warrants as follows:

         (a) The execution and delivery of this Amendment and the performance of
this Amendment, the Loan Agreement as amended hereby and each of the other Loan
Documents, and the transactions contemplated hereby and thereby, have been
authorized by all necessary corporate actions of the Borrower. This Amendment,
the Loan Agreement as amended hereby and each of the other Loan Documents
constitute the legal, valid and binding obligations of the Borrower, enforceable
against the Borrower in accordance with their respective terms.

         (b) The Borrower has all requisite corporate power and authority to
execute, deliver and perform its obligations under this Amendment, the Loan
Agreement as amended hereby and each of the other Loan Documents. Neither the
authorization, execution, delivery or performance by the Borrower of this
Amendment nor the performance of the Loan Agreement as amended hereby or any
other Loan Document nor the performance of the transactions contemplated hereby
or thereby violates or will violate any provision of the corporate charter or

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by-laws of the Borrower, or does or will, with the passage of time or the giving
of notice or both, result in a breach of or a default under, or require any
consent under or result in the creation of any lien, charge or encumbrance upon
any property or assets of the Borrower pursuant to, any material instrument,
agreement or other document to which the Borrower is a party or by which the
Borrower or any of its properties may be bound or affected.

         (c) The execution and delivery by the Borrower of this Amendment and
the performance by the Borrower of the Loan Agreement as amended hereby and the
Loan Documents do not and will not violate any provision of law or regulation
applicable to the Borrower, or any writ, order or decree of any court or
governmental or regulatory authority or agency applicable to the Borrower.

     Section 3. Loan Documents. This Amendment shall be a Loan Document for all
purposes.

     Section 4. Conditions to Effectiveness. The effectiveness of this Amendment
is conditioned on the following:

         (a) The Borrower and the Lender shall each have executed and delivered
a counterpart of this Amendment;

         (b) The representations and warranties contained in Article V of the
Loan Agreement shall be true and correct in all material respects as of the date
hereof as though made on and as of the date hereof;

         (c) No Default or Event of Default under the Loan Agreement shall have
occurred and be continuing; and

         (d) The Lender shall have received, in form and substance satisfactory
to it:

               (i) an opinion of independent counsel to the Borrower with
          respect to this Amendment;

               (ii) a certificate as to the Borrower's legal existence and good
          standing under the laws of The Commonwealth of Massachusetts; and

               (iii) a certificate of the Borrower's Clerk as to (x) no changes
          in its charter documents and by-laws as amended, (y) corporate votes
          authorizing the execution and delivery of this Amendment and (z)
          incumbency of the officers authorized to execute this Amendment on
          behalf of the Borrower;

           (e) The conditions set forth in clauses (b) - (e) of Section 6.1 of
the Loan Agreement shall have been met as of the date hereof, provided that for
purposes thereof and Section 5.5 of the Loan Agreement, the "Balance Sheet Date"
shall mean September 30, 1999

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and the financial statements referred to therein shall mean the unaudited
statements for the period ended September 30, 1999, that have been furnished to
the Lender.

     Section 5. Miscellaneous

         (a) On and after the date hereof, each reference in the Loan Agreement
to "this Agreement" or words of like import shall mean and be deemed to be a
reference to the Loan Agreement as amended hereby.

         (b) Except as amended and modified hereby, the Loan Agreement is in all
respects ratified and confirmed as of the date hereof, and the terms, covenants
and agreements therein shall remain in full force and effect.

         (c) This Amendment and the modifications to the Loan Agreement set
forth herein shall be deemed to be a document executed under seal and shall be
governed by and construed in accordance with the laws of The Commonwealth of
Massachusetts.

         (d) This Amendment may be executed in counterparts, each of which shall
be deemed an original, but all of which together shall constitute one and the
same document.

     IN WITNESS WHEREOF, the parties have caused this Amendment to be duly
executed as of the date and the year first above written.

                                                      MKS INSTRUMENTS, INC.

                                                      By: William P. Donlan
                                                      Title: Treasurer


                                                      BANKBOSTON, N.A.

                                                      By: /s/ Sharon A. Stone
                                                      Title: Director


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and the financial statements referred to therein shall mean the unaudited
statements for the period ended September 30, 1999, that have been furnished to
the Lender.

     Section 5. Miscellaneous

         (a) On and after the date hereof, each reference in the Loan Agreement
to "this Agreement" or words of like import shall mean and be deemed to be a
reference to the Loan Agreement as amended hereby.

         (b) Except as amended and modified hereby, the Loan Agreement is in all
respects ratified and confirmed as of the date hereof, and the terms, covenants
and agreements therein shall remain in full force and effect.

         (c) This Amendment and the modifications to the Loan Agreement set
forth herein shall be deemed to be a document executed under seal and shall be
governed by and construed in accordance with the laws of The Commonwealth of
Massachusetts.

         (d) This Amendment may be executed in counterparts, each of which shall
be deemed an original, but all of which together shall constitute one and the
same document.

     IN WITNESS WHEREOF, the parties have caused this Amendment to be duly
executed as of the date and the year first above written.

                                                      MKS INSTRUMENTS, INC.

                                                      By:
                                                      Title:


                                                      BANKBOSTON, N.A.

                                                      By: /s/ Sharon A. Stone
                                                      Title: Director


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